waiig15/parser/parser.go

package parser

import (
	"fmt"

	"strconv"

	"code.osinet.fr/fgm/waiig15/ast"
	"code.osinet.fr/fgm/waiig15/lexer"
	"code.osinet.fr/fgm/waiig15/token"
)

// Precedence constants.
const (
	_ int = iota
	LOWEST
	EQUALS      // ==
	LESSGREATER // > or <
	SUM         // +
	PRODUCT     // *
	PREFIX      // -X or !X
	CALL        // myFunction(X)
)

var precedences = map[token.TokenType]int{
	token.EQ:       EQUALS,
	token.NOT_EQ:   EQUALS,
	token.LT:       LESSGREATER,
	token.GT:       LESSGREATER,
	token.PLUS:     SUM,
	token.MINUS:    SUM,
	token.SLASH:    PRODUCT,
	token.ASTERISK: PRODUCT,
}

type (
	prefixParseFn func() ast.Expression
	infixParseFn  func(ast.Expression) ast.Expression
)

// Parser implements the parsing mechanism top-level layer.
type Parser struct {
	l      *lexer.Lexer
	errors []string

	curToken  token.Token
	peekToken token.Token

	prefixParseFns map[token.TokenType]prefixParseFn
	infixParseFns  map[token.TokenType]infixParseFn
}

// New returns a new Parser instance with the first two parser tokens already
// loaded.
func New(l *lexer.Lexer) *Parser {
	p := &Parser{
		l:      l,
		errors: []string{},
	}

	p.prefixParseFns = make(map[token.TokenType]prefixParseFn)
	p.registerPrefix(token.IDENT, p.parseIdentifier)
	p.registerPrefix(token.INT, p.parseIntegerLiteral)
	p.registerPrefix(token.BANG, p.parsePrefixExpression)
	p.registerPrefix(token.MINUS, p.parsePrefixExpression)

	p.infixParseFns = make(map[token.TokenType]infixParseFn)
	for _, tok := range []token.TokenType{
		token.ASTERISK,
		token.EQ,
		token.GT,
		token.LT,
		token.MINUS,
		token.NOT_EQ,
		token.PLUS,
		token.SLASH,
	} {
		p.registerInfix(tok, p.parseInfixExpression)
	}

	// Read two tokens, so curToken and peekToken are both set
	p.nextToken()
	p.nextToken()

	return p
}

func (p *Parser) nextToken() {
	p.curToken = p.peekToken
	p.peekToken = p.l.NextToken()
}

// Is the current token in the parser of the given type ?
func (p *Parser) curTokenIs(t token.TokenType) bool {
	return p.curToken.Type == t
}

// Is the next token in the parser of the given type ? Don't consume it.
func (p *Parser) peekTokenIs(t token.TokenType) bool {
	return p.peekToken.Type == t
}

// Is the next token in the parser of the given type ? If it is, consume it,
// else don't.
func (p *Parser) expectPeek(t token.TokenType) bool {
	if p.peekTokenIs(t) {
		p.nextToken()
		return true
	}

	p.peekError(t)
	return false
}

// Errors is a getter for Parser.errors.
func (p *Parser) Errors() []string {
	return p.errors
}

// Log a mismatch error on the peek token type in the parser instance.
//
//   - t is the type of token that was expected
func (p *Parser) peekError(t token.TokenType) {
	msg := fmt.Sprintf("expected next token to be %s, got %s instead",
		t, p.peekToken.Type)
	p.errors = append(p.errors, msg)
}

func (p *Parser) noPrefixParseFnError(t token.TokenType) {
	msg := fmt.Sprintf("no prefix parse function for %s found", t)
	p.errors = append(p.errors, msg)
}

// ParseProgram is the outermost parsing logic, accumulating statements in a
// Program instance and returning that instance once parsing is done.
func (p *Parser) ParseProgram() *ast.Program {
	defer untrace(trace("ParseProgram"))
	program := &ast.Program{
		Statements: []ast.Statement{},
	}
	for !p.curTokenIs(token.EOF) {
		stmt := p.parseStatement()
		if stmt != nil {
			program.Statements = append(program.Statements, stmt)
		}
		p.nextToken()
	}

	return program
}

func (p *Parser) parseStatement() ast.Statement {
	defer untrace(trace("parseStatement"))
	switch p.curToken.Type {
	case token.LET:
		return p.parseLetStatement()
	case token.RETURN:
		return p.parseReturnStatement()
	default:
		return p.parseExpressionStatement()
	}
}

func (p *Parser) parseLetStatement() *ast.LetStatement {
	defer untrace(trace("parseLetStatement"))
	stmt := &ast.LetStatement{
		Token: p.curToken,
	}

	// Let statement starts with an IDENT token, so if next token is not an
	// IDENT, the next statement cannot be a Let statement.
	if !p.expectPeek(token.IDENT) {
		return nil
	}

	stmt.Name = &ast.Identifier{
		Token: p.curToken,
		Value: p.curToken.Literal,
	}

	// The previous expectPeek() call fetched the next token, so we should now
	// be on the assignment.
	if !p.expectPeek(token.ASSIGN) {
		return nil
	}

	// Skip the expression for now, progress to the semicolon terminating the
	// statement.
	for !p.curTokenIs(token.SEMICOLON) {
		p.nextToken()
	}

	return stmt
}

func (p *Parser) parseReturnStatement() *ast.ReturnStatement {
	defer untrace(trace("parseReturnStatement"))
	stmt := &ast.ReturnStatement{
		Token: p.curToken,
	}

	// There should be an expression to consume here.
	p.nextToken()

	// Skip the expression for now, progress to the semicolon terminating the
	// statement.
	for !p.curTokenIs(token.SEMICOLON) {
		p.nextToken()
	}

	return stmt
}

func (p *Parser) parseExpressionStatement() *ast.ExpressionStatement {
	defer untrace(trace("parseExpressionStatement"))
	stmt := &ast.ExpressionStatement{
		Token: p.curToken,
	}

	stmt.Expression = p.parseExpression(LOWEST)

	// Semicolons are optional to help use REPL input.
	if p.peekTokenIs(token.SEMICOLON) {
		p.nextToken()
	}

	return stmt
}

func (p *Parser) parseExpression(precedence int) ast.Expression {
	defer untrace(trace("parseExpression"))
	prefix := p.prefixParseFns[p.curToken.Type]
	if prefix == nil {
		p.noPrefixParseFnError(p.curToken.Type)
		return nil
	}
	leftExp := prefix()

	for !p.peekTokenIs(token.SEMICOLON) && precedence < p.peekPrecedence() {
		infix := p.infixParseFns[p.peekToken.Type]
		if infix == nil {
			return leftExp
		}

		p.nextToken()

		leftExp = infix(leftExp)
	}

	return leftExp
}

// Look forward for the precedence of the next token without advancing.
func (p *Parser) peekPrecedence() int {
	if precedence, ok := precedences[p.peekToken.Type]; ok {
		return precedence
	}

	return LOWEST
}

// Return the precedence for the current token without advancing.
func (p *Parser) curPrecedence() int {
	if precedence, ok := precedences[p.curToken.Type]; ok {
		return precedence
	}

	return LOWEST
}

// parseIdentifier does not advance the tokens or call nextToken, and this is
// important.
func (p *Parser) parseIdentifier() ast.Expression {
	return &ast.Identifier{
		Token: p.curToken,
		Value: p.curToken.Literal,
	}
}

func (p *Parser) parseIntegerLiteral() ast.Expression {
	defer untrace(trace("parseIntegerLiteral"))
	lit := &ast.IntegerLiteral{
		Token: p.curToken,
	}

	// Base 0 allows straight interpretation of octal 0755 or hex 0xABCD.
	value, err := strconv.ParseInt(p.curToken.Literal, 0, 64)
	if err != nil {
		msg := fmt.Sprintf("could not parse %q as integer",
			p.curToken.Literal)
		p.errors = append(p.errors, msg)
		return nil
	}

	lit.Value = value

	return lit
}

func (p *Parser) parsePrefixExpression() ast.Expression {
	defer untrace(trace("parsePrefixExpression"))
	expression := &ast.PrefixExpression{
		Token:    p.curToken,
		Operator: p.curToken.Literal,
	}

	// Consume the operator token to progress to the prefixed expression.
	p.nextToken()

	// The precedence is now that of the prefix operator instead of the lowest.
	expression.Right = p.parseExpression(PREFIX)

	return expression
}

func (p *Parser) parseInfixExpression(left ast.Expression) ast.Expression {
	defer untrace(trace("parseInfixExpression"))
	expression := &ast.InfixExpression{
		Token:    p.curToken,
		Operator: p.curToken.Literal,
		Left:     left,
	}

	precedence := p.curPrecedence()
	p.nextToken()
	expression.Right = p.parseExpression(precedence)

	return expression
}

func (p *Parser) registerPrefix(tokenType token.TokenType, fn prefixParseFn) {
	p.prefixParseFns[tokenType] = fn
}

func (p *Parser) registerInfix(tokenType token.TokenType, fn infixParseFn) {
	p.infixParseFns[tokenType] = fn
}